1. Field of the Invention
The present invention relates to a belt apparatus having an endless belt which is wound around and movably driven by a plurality of support rollers, and an image forming apparatus having the belt apparatus.
2. Description of the Background Art
The above described belt apparatus is being widely used for mechanical apparatus in various technical fields. For example, in copiers, printers, facsimile machines, or image forming apparatuses which are configured as a combination unit of these mechanisms, an endless belt of such a belt apparatus is placed facing multiple photosensitive elements to transfer toner images, which are formed on each photosensitive element and have colors different from each other, one upon another onto a recording medium which is supported and conveyed by the driven endless belt described above, and the toner images are fixed on the recording medium by a fixing apparatus. Such a belt apparatus is disclosed in, for example, Japanese Patent No. 2,608,080 (PAGE 3, FIG. 1).
Also known is an image forming apparatus in which toner images formed on each photosensitive element and having colors different from each other are transferred one upon another onto an endless belt of a belt apparatus, and thereafter the toner images are transferred onto a recording medium and fixed on the recording medium by a fixing apparatus.
In conventional belt apparatuses such as described above, one of the support rollers having the endless belt wound thereon is configured as a driving roller, and the driving roller is rotatably driven by a driving source to move the endless belt. However, an endless belt inevitably has thick part and thin part due to the nature of its manufacturing method resulting in uneven thickness distribution in the circumferential direction, and therefore adopting the above described configuration will cause speed fluctuations of the endless belt.
When such speed fluctuations occur in the endless belt applications, various problems will result. For example, when the belt apparatus is adopted in an image forming apparatus described above, the fluctuations of the endless belt will cause a problem that color shifts occur in a superposed image, which is transferred onto a recording medium or the endless belt from a plurality of photosensitive elements, thereby degrading the image quality.